Spectrofluorimetry

Dr. Chris

Phayao University, March 2016

Introduction

• Absorption of uv/visible radiation causes transition of electrons from ground state (low energy) to excited state (high energy).

• As excited state is not stable, excess energy is lost by

– Collision deactivation

– Emission of radiation (Photo Luminescence)

• Emission Spectroscopy : emission of radiation is studied.

Understanding the terms…….. • Singlet ground state : state in which electrons in a

molecule are paired. [ ]

• Singlet excited state: state in which electrons are unpaired but of opposite spins. [ ]

• Triplet state: state in which unpaired electrons of same spin are present. [ ]

• Excitation process: absorption of energy or light followed by conversion from ground state to excite state.

• Relaxation process: process by which atom or molecule losses energy & returns to ground state.

Photo Luminescence

• Light without heat or cold light

• Basically of 2 types

– Fluorescence: part of energy is lost due to vibrational transitions and remaining energy is emitted as uv/visible radiation of longer wavelength than incident light.

– Phosphorescence: under favorable conditions, excited singlet state undergo transition to triplet state. Emission of radiation when e- undergo transition from triplet state to ground state.

Flourescence – emission at higher wavelength (lower energy) than excitation

https://www.youtube.com/watch?v=SGFlr1jFNBM

http://www.chem.uci.edu/~dmitryf/manuals/Fundamentals/Fluorescence%20Excitation%20and%20Emission%20Fundamentals.pdf

Instrumentation

Fixed λ => EXCITATION

Fixed λ => EMISSION

Excitation and Emission

For the Excitation spectra: Detection wavelength is fixed, the excitation wavelength is varied For the Emission spectra: The excitation wavelength is fixed, the detection wavelength is varied

Classification

• Based on the wavelength of emitted radiation when compared to absorbed radiation

– Stokes fluorescence: wavelength of emitted radiation is longer than absorbed radiation

– Anti-stokes’s fluorescence: wavelength of emitted radiation is shorter than absorbed radiation.

– Resonance fluorescence: wavelength of emitted radiation is equal to that of absorbed radiation.

Actors affecting fluorescence intensity

• Conjugation: molecule must have conjugation ( π electron) so that uv/vis radiation can be absorbed

• Nature of substituent groups:

– e- donating groups like NH2, OH groups enhance fluorescence.

– e- withdrawing groups like NO2, COOH reduce fluorescence.

• Fluorescent intensity is directly proportional to concentration.

• Increase in viscosity leads to decreased collisions of molecules there by increasing fluorescent intensity.

• More rigid the structure of molecule, more the intensity of fluorescence.

• Increase in temp leads to increased collisions b/w molecules decreasing fluorescent intensity.

• Presence of O2 decreases the fluorescence and so de-aerated solutions must be used.

• Source of light

– Mercury vapour lamp : Hg vapour in high pressure (8 atm) gives intense lines on continuous background above 350nm.

– Xenon arc lamp: gives more intense radiation.

– Tungsten lamp: used if excitation has to be done in vis region.

• Filters and monochromators

– In fluorimeter 10 filter ( absorb vis radiation and transmit uv radiation) and 20 filter (absorb uv radiation and transmit vis radiation) are present.

– In spectrofluorimeters, excitation monochromators and emission monochromator are present.

• Sample cells

– Sample cells are cylindrical or polyhedral made up of colour corrected fused glass & path length normally 10mm to 1cm.

• Detectors

– Photo voltaic cell, photo tubes or photo multiplier tubes can be used.

Advantages

• More sensitive when compared to other absorption techniques. Concentrations as low as μg/ml or ng/ml can be determined.

• Precision upto 1% can be achieved easily

• As both excitation & emission wave lengths are characteristic it is more specific than absorption methods.

Example: Naphtalene in water

Uv/vis spectrum https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/spectrum.htm

Flourescence spectrum https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-21-20-24219&id=268663

Minerals: Minerals: Cr 3+ emissions from Al2O3 sapphires Absorbs blue and green light Red color Emits red color

http://www.eso.org/~rfosbury/Article%20from%20Journal2013.pdf

Applications of Spectrofluorimetry

• Determination of Organic substances – Plant pigments, steroids, proteins, naphthols etc can be

determined at low concentrations. – Generally used to carry out qualitative as well as

quantitative analysis for a great aromatic compounds present in cigarette smoking, air pollutant concentrates & automobile exhausts.

• Determination of inorganic substances • Extensively used in the field of nuclear research for the

determination of uranium salts. • Determination of vitamin B1 (thiamine) in food samples

like meat cereals etc. • Determination of Vitamin B2 (riboflavin). This method

is generally used to measure the amount of impurities present in the sample.

• Most important applications are found in the analyses of food products, pharmaceuticals, clinical samples and natural products.

• Fluorescent indicators:

– Intensity and colour of the fluorescence of many substances depend upon the pH of solutions. These are called as fluorescent indicators and are generally used in acid base titrations.

– Eg: Eosin – pH 3.0-4.0 – colourless to green

– Fluorescein – pH 4.0-6.0 – colourless to green